1. Field of the Invention
The present invention relates to a magnetic field-generating device for use in a medical nuclear magnetic resonance tomograph (hereinafter referred to as an NMR-CT) and which employs permonent Magnets.
2. Description of the Prior Art
A conventional magnetic field-generating device for use in an NMR-CT is shown in FIG. 7. It includes a pair of permanent magnet components 1, pole pieces 2 which are respectively fixed to one end of each of the permanent magnet components 1, and a yoke 3 coupling the other ends of the permanent magnet components 1. A static magnetic field is generated within an air gap 4 formed between the pole pieces 2.
The present inventor has proposed magnetic field generating devices which enable the degree of uniformity of the magnetic field generated in the gap to be improved. These include devices wherein the pole pieces have annular protrusions at the outer peripheries thereof, and devices wherein the pole pieces have a convex protrusion at the central portion. See U.S. Pat. No. 4,672,346 and EP 0161782 A1.
In either of the the above-described magnetic field-generating devices, the pole pieces each comprise a plate-like mass formed by plaining a magnetic material such as carbon steel, i.e., the pole pieces have a solid structure.
A pole piece of this type is effective in causing the magnetic flux generated by the permanent magnetic component to act uniformly within the gap. However, the permeability in the path of the magnetic flux within such a pole piece 2 is equal in all directions, and part of the magnetic flux therefore leaks from the sides of the pole piece into the surrounding yoke.
Therefore, in order to cause the magnetic flux generated by the permanent magnet component to act effectively within the gap so as to obtain a desired magnetic field intensity, the size of the permanent magnetic component must be made large. This prevents a desired reduction in the size of the NMR-CT.